Optimal subpixel arrangement for displays with more than three primary colors

ABSTRACT

A display comprising a plurality sub-pixel display elements respectively displaying different colors, arranged into a plurality of single-row pixels, wherein for adjacent pixels, each sub-pixel display element associated with the first pixel bears a minimal spatial distance to a respective sub-pixel display element associated with the second pixel that represents a color having a maximum color distance with respect to the color represented by such sub-pixel display element, as compared to the color distance between the color represented by the sub-pixel display element and other colors.

The present invention relates to a display comprising a plurality offirst, second, third and fourth display elements, which are controllableto display first, second, third and fourth primary colors, respectively.

Vision is the sense, mediated by the eyes, by which the qualities of anobject (such as color, luminosity, shape and size) constituting itsappearance are perceived.

Color is defined as an attribute of visual perception consisting of anycombination of chromatic and achromatic content. This attribute can bedescribed by chromatic color names such as yellow, orange, brown, red,pink, green, blue, purple, etc., or by achromatic color names such aswhite, grey, black, etc., and qualified by bright, dim, light, dark,etc., or by combinations of such names.

A perceived color depends on the spectral distribution of the colorstimulus, on the size, shape, structure and surround of the stimulusarea, on the state of adaptation of the observer's visual system, and onthe observer's experience of the prevailing and similar situations ofobservations.

The unrelated attributes of color are brightness, hue and saturation.Brightness is the attribute of a visual sensation according to which anarea appears to emit more or less light. Hue is an attribute of a visualsensation according to which an area appears to be similar to one of theperceived colors, e.g. red, yellow, green, and blue, or to a combinationof them. Saturation is the colorfulness, chromaticness, of an areajudged in proportion to its brightness.

The related attributes of color are lightness, colorfulness and chroma.Lightness is defined as the brightness of an area judged relative to thebrightness of a similarly illuminated area that appears to be white orhighly transmitting. Colorfulness is an attribute of a visual sensationaccording to which the perceived color of an area appears to be more orless chromatic. Chroma is defined as the colorfulness, chromaticness ofan area judged as a proportion of the brightness of a similarlyilluminated area that appears white or highly transmitting.

In the retina of the eye there are three different types of lightsensors. These sensors are called the L, M and S cones, which aresensitive to light with long (L), medium (M) and short (S) wavelengths,respectively. Each type of sensor is connected with neurones to thebrain. When light falls on a cone it will start to send pulses to thebrain when it is sensitive to the wavelength of the light. FIG. 1 showsthe spectral sensitivities of L, M and S cones in the human eye. Themore light falls on the cones the quicker they will send pulses (“firespikes”) to the brain.

The color of the light that enters the eye is determined by the relativeamount of pulses that each of the three types of cones sends to thebrain. Blue light (wavelength approximately 400-450 nm), for example,results in more spikes from the S cones than from the L cones or the Mcones.

Because the human eye has only three types of cones, there are a numberof different light spectra that give the same color sensation. Forexample, sunlight and the light from a fluorescent lamp are bothperceived as white in color, but whereas the sunlight has a very broadspectrum with about equal intensity for each wavelength, the fluorescentlamp has a spectrum with only a few peaks. This effect of differentlight spectra giving the same color sensation is called metamerism, andtwo spectra which give the same color sensation are called metamers.

Another effect of having only three types of cones is that differentcolors can be made by adding together the light of two light sourceswhile varying the relative intensity of these light sources. If redlight and green light are mixed, it may be perceived as yellow. If afirst light source emitting red light is set to full intensity and asecond light source emitting green light is set to zero intensity, andthe intensity of the green light is increased while the intensity of thered light is decreased, the color changes from red, to orange, toyellow, and finally to green.

Displays use this principle to make many colors with only three primarycolors; usually red, green and blue.

In order to predict the color sensation that we get from the light thatenters our eyes, a number of models have been developed. One of thesemodels, which is most commonly known and which is standardised by theCIE (Commission Internationale d'Éclairage—International Commission onIllumination) is the CIE 1931 model. It defines three spectral matchingfunctions for the standard observer that can be used to calculate thetri-stimulus values X, Y, and Z, respectively, for a light with acertain spectrum. From these tri-stimulus values the chromaticitycoordinates x and y can be calculated as follows:

$\begin{matrix}{x = \frac{X}{X + Y + Z}} & (1) \\{y = \frac{Y}{X + Y + Z}} & (2)\end{matrix}$

Y is related to the perceptual attribute brightness, the x and ycoordinates determine the chromaticity, where x is the red-green axisand y is the yellow-blue axis.

The relation between colors (while ignoring the intensity, Y) can now beplotted in a two-dimensional chromaticity diagram, such as FIG. 2. Itshows the chromaticity coordinates of the spectral colors by the curvedline and indicates the corresponding wavelengths in nanometers (nm).Chromaticity coordinates for all visible colors are on the horseshoeshaped area inside the curved line. The straight line at the bottom ofthe chart (the purple line) connects the red and the blue spectralcolors, so that non-spectral colors obtained by mixing red and blue(e.g. purple, violet, etc.) are located along this line. Thechromaticity coordinate of a white object in daylight is designated D inFIG. 2. The direction and the distance of a certain point in thechromaticity diagram to the white point determine its hue andsaturation.

As mentioned previously, mixing the light of two colors can create a newcolor. The chromaticity coordinate of this new color is on an imaginarystraight line between the two colors. Mixing green (G) and cyan (C) willfor instance give a color whose chromaticity coordinate is on the brokenline 21 between G and C as given in FIG. 2. By adding a third color,e.g. red (R), all colors within an imaginary triangle, spanned by R, G,and C can be made. By mixing light of six different primary colors (e.g.R, Y, G, C, B, M), all colors with chromaticity coordinates in the patchR, Y, G, C, B, M, i.e. inside a polygon, the corners of which are R, Y,G, C, B, and M, can be made.

The chromaticity diagram only shows the proportions of tristimulusvalues; hence bright and dim colors having the same tristimulusproportions belong to the same point. For this reason, the illuminantpoint D also represents grey colors; and orange and brown colors, forexample, tend to plot at similar positions to each other.

The subject matter of color vision is further elucidated in e.g. Roy S.Berns, Fred W. Billmeyer, and Max Saltzman; Billmeyer and Saltzman'sPrinciples of Color Technology, 3rd Edition; ISBN 0-471-19459-X, herebyincorporated in its entirety by this reference.

The present invention relates to the field of displays in general, andin particular to liquid crystal displays (LCD), cathode ray tube (CRT)displays, flat intelligent tube (FIT) displays, light emitting diode(LED) displays, all of which will be explained briefly in the following,as well as to plasma display panels (PDP), PolyLED displays, organiclight emitting displays (OLED), field emission displays (FED), and foildisplays.

In prior art, liquid crystal displays have proven themselves suitablefor various applications which necessitate compactness and low powerconsumption. A liquid crystal display (LCD) is a flat panel displaydevice having the advantages of small bulk, small thickness and lowpower consumption.

LCDs have been used in connection with portable devices such as mobiletelephones, portable computers, electronic calendars, electronic books,televisions or video game controls and various other office automationequipment and audio/video machinery, etc.

LCDs control an electric field which is applied to a liquid crystalmaterial having a dielectric anisotropy to modulate light, therebydisplaying a picture or an image, all in a fashion that is known per seas is recognized by those skilled in the art. Unlike display devicesthat generate light internally—such as electro luminescence (EL)devices, cathode ray tubes (CRT) and light emitting diodes (LED)—LCDsuse an external light source.

Normally, an LCD display is designed as a liquid crystal panel,comprising a matrix of essentially rectangular display elements (pixels)which are controllable to transmit or reflect light depending on theproperties of the liquid crystal mixture, which generally is injectedbetween two transparent substrates, the display in addition comprisingrow and column conductors for supplying voltages to selected parts ofthe display, via associated electronics such as row and column drivers,as will be recognized by the skilled man.

LCD devices are broadly classified into transmissive type devices andreflective type devices, depending on the method of utilizing light.Transmissive type LCDs include a back light unit for supplying light tothe liquid crystal panel.

Light emitting diodes (LED) have been used to create big screen devicessuch as jumbo-TVs. Depending on the desired pixel size, a number of red,green and blue light emitting diodes may be grouped together to form asingle display element, corresponding to a pixel in an LCD display. Suchdisplay elements are subsequently arranged in a rectangular matrix andconnected to necessary electronics as will be recognized by the skilledman.

FIG. 3 is a schematic illustration of the fundamental principle of thecathode ray tube (CRT), which is comprised in many TVs in use today aswell as in many other display devices. A cathode 31, for instance aheated filament, is arranged inside a glass tube 32, in which a vacuumhas been created. Electrons are naturally released from the heatedcathode 31 and migrate into the tube 32. An anode 33 attracts theelectrons, which are released from the cathode 31, thus forming a beamor ray of electrons 34. In the cathode ray tube 32 of a television set,the beam of electrons 34 is focused by a focusing anode 33 into a tightbeam and then accelerated by an accelerating anode 35. The beam ofelectrons 34 flies through the vacuum inside the tube 32 and hits a flatscreen 36 at the other end of the tube 32. This screen 36 is coated withphosphor 37, which glows when struck by the electron beam 34. Aconductive coating inside the tube soaks up the electrons which pile upat the screen-end of the tube.

In order to provide means to guide the beam 34, the tube 32 in a typicalCRT display device is wrapped in steering coils 38, 39. The steeringcoils 38, 39 are simply copper windings, which are able to createmagnetic fields inside the tube, and the electron beam 34 responds tothe fields. A first set of coils 38 creates a magnetic field that movesthe electron beam vertically, while a second set of coils 39 moves thebeam horizontally. By controlling the voltages applied to the coils 38,39, the electron beam 34, can be positioned at any point on the screen36.

A color CRT display comprises three electron beams, typically denoted asthe red, green and blue beams, which move simultaneously across thescreen. Instead of the single sheet of phosphor which is arranged at thescreen in black-and-white CRT display devices, the screen in a color CRTdisplay is coated with red, green and blue phosphors arranged in dots orstripes. On the inside of the tube, very close to the phosphor coating,there is arranged a thin metal screen, i.e. the shadow mask. This maskis perforated with very small holes that are aligned with the phosphordots (or stripes) on the screen.

A red dot may be created by firing the red beam at the red phosphor,whereas green and blue dots are created in a corresponding fashion. Tocreate a white dot, red, green and blue beams are firedsimultaneously—the three colors mix together to create white. To createa black dot, all three beams are turned off as they scan past the dot.All other colors on a color CRT display are combinations of red, greenand blue. CRT displays are typically time sequential displays, whichimplies that an image is built up by repeatedly scanning the beam(s)over the screen, whereupon an image is displayed, all in a manner knownper se as will be appreciated by the skilled man.

The Flat Intelligent Tube (sometimes referred to as FIT or FIT) is a newcathode ray tube (CRT) technology without a shadow mask. The primaryfunction of the shadow mask, i.e. color selection, is managed by anelectronic control system that guides the electron beams over thecorrect phosphor lines. The position of the beams is detected by meansof dedicated structures on the faceplate.

FIG. 4 is a simplified representation of the tracking principle in aFIT. In the FIT, the beams 34 are scanned along horizontal phosphorlines 41, in contrast to mask-less CRTs of the index type developed inthe past in which a single beam was scanned perpendicularly to thevertical phosphor lines. The FIT approach is quite similar to that of aCD-player wherein a laser beam is guided over a spiral by means of atracking system. The beam 34 is scanned along a horizontal phosphor line41 and any deviation from this line is corrected by means of a feedbacksystem. On tracks situated above and below each phosphor line 41,position detectors 42 are present (e.g. conducting stripes that measurethe current). A control system 43, fed by information from thesedetectors 42, drives correction coil(s) 44 in such a way that the beamtrajectories coincide with the phosphor lines 41.

In the CRT and FIT displays, the phosphor dots or stripes constitute thedisplay elements, which accordingly are controllable to emit lighthaving a predetermined wavelength (color).

In prior art RGB color displays, the displayable color gamut is limitedto a color triangle, which is spanned by three primary colors, e.g. red,green and blue (as illustrated in FIG. 2). Colors outside this colortriangle, e.g. gold and turquoise (in a case where the primary colorsare red, green and blue), cannot be displayed and are consequentlyclipped towards colors that can be displayed, e.g. more unsaturatedyellow and more bluish green. It is known that adding one or moreadditional primary colors to the three primary colors used in mostpresent applications offers a possibility to expand the displayablecolor gamut.

Spatial resolution is the ability of a display system to display twoobjects that are close to each other as separate dots. For all displaytypes that cannot project various color pixels on top of each other, theaddition of a sub-pixel with a different-colored primary yields areduction in the spatial resolution of the display if the number ofsub-pixels remains equal.

The smallest switching element is the sub-pixel. If the sub-pixels aremade smaller, there can be four sub-pixels in one pixel having the samesize as a pixel with three sub-pixels. This however is costly andgenerally speaking resolution decreases as the amount of sub-pixelsincreases. If, on the other hand, the size of sub-pixels is keptconstant and four, instead of three, sub-pixels are used to form apixel, the pixel resolution will decrease.

Furthermore, the addition of more than three colors may result in errorsrelating to color, luminance and image homogeneity.

It is accordingly a disadvantage that the addition of one or moreprimary colors in matrix displays results in a reduction of the overallimage quality.

It is an object of the invention to provide a display, wherein thereduction of the overall image quality, particularly the color andluminance errors, resulting from the addition of one or more primarycolors, is limited.

It is another object of the invention to provide a display, wherein anoptimal arrangement of the display elements results in an increasedhomogeneity in color and luminance.

It is another object of the invention to provide a display, whereinblack and white transitions on sub-pixel level without color artefactscan be generated using sub-pixel algorithms.

The present invention relates to a display comprising a plurality offirst, second, third and fourth display elements, which are controllableto display first, second, third and fourth primary colors, respectively,characterized in that a particular pair of said display elements, whichrepresents a maximum color distance compared to other pairs of saiddisplay elements, is arranged such that there is a minimal spatialdistance between the elements of said particular pair or that theelements of said particular pair are situated next to each other.

The measures as defined in claims 2-6 have the advantages that theyconstitute increasingly optimal solutions to the problem of maximizingcolor distance while minimizing the spatial distance between displayelements.

The measures as defined in claims 7-12 have the advantage that thesecolors are particularly suitable for use in color displays.

The measure as defined in claim 13 has the advantage of increased imagequality due to an improved distribution of the luminance.

These and other aspects of the invention will be apparent from andelucidated with reference to the embodiments described hereinafter.

Essentially speaking, the invention relates to a new and innovativemethod of arranging the display elements of a matrix display having fouror more primary colors. According to the invention, the optimalarrangement will result in the best homogeneity in color and luminance.In addition, the arrangement according to the invention will limit thecolor and luminance errors.

An important aspect of the invention is the fact that although thedisplay resolution on pixel level is lower, black and white transitionson sub-pixel level can be made without color artefacts as illustrated inFIG. 14 and FIG. 15 when using so-called sub-pixel algorithms.

FIG. 1 shows the spectral sensitivities of L, M and S cones in the humaneye.

FIG. 2 is a chromaticity diagram.

FIG. 3 is a schematic illustration of the fundamental principle of acathode ray tube (CRT).

FIG. 4 is a simplified representation of the tracking principle in aFlat Intelligent Tube (FIT).

FIG. 5 is a schematic illustration of the stripe-arrangement ofsub-pixels in prior art color displays.

FIG. 6 is a schematic illustration of the mosaic-arrangement ofsub-pixels in prior art color displays.

FIG. 7 is a schematic illustration of the delta-arrangement ofsub-pixels in prior art color displays.

FIG. 8 is a schematic illustration of an RGBYMC-stripe-arrangement ofsub-pixels in a six color display.

FIG. 9 is a schematic illustration of the arrangement of sub-pixels in asix-color display, wherein RGB-stripes and YMC-stripes alternate insubsequent rows.

FIG. 10 is a schematic illustration of the arrangement of sub-pixels ina six-color display, wherein RGB-stripes and YMC-stripes alternate insubsequent rows and columns.

FIG. 11 is a schematic illustration of the arrangement of sub-pixels ina six-color display according to a first embodiment of the invention,wherein RGB-stripes and CMY-stripes alternate in subsequent rows.

FIG. 12 is a schematic illustration of the arrangement of sub-pixels ina six-color display according to a second embodiment of the invention,wherein RGB-stripes and CMY-stripes alternate in subsequent rows andcolumns.

FIG. 13 is a schematic illustration of the arrangement of sub-pixels ina six-color display according to a third embodiment of the invention,wherein RGBCMY-stripes and CMYRGB-stripes are arranged alternatingly.

FIG. 14 is a schematic illustration of the smallest possible whitedisplay element in a prior art RGB display.

FIG. 15 is a schematic illustration of the smallest possible whitedisplay element in a display with an RGB/CMY sub-pixel arrangementaccording to the first embodiment of the invention.

Prior art multicolor displays comprise displays with red, green and blueprimary colors; and an additional primary color such as yellow or white.In an LCD with four primary colors, each pixel is built up out of four“sub-pixels”: e.g. a red, a green, a blue and a yellow sub-pixelconstitute a pixel.

When selecting an additional primary color, its impact on the luminanceand the color gamut of a display should be taken into account. Whenconsidering the luminance alone, a primary color with a high luminance,such as those in the triangle yellow-white-green, appears desirable.Regarding the color gamut, with a view to extending the color gamut asmuch as possible, a highly saturated yellow, cyan or magenta would bepreferred.

Yellow is furthermore a color which carries much brightness, andtherefore its absence is easily detected; and this is why adding moresaturated yellow colors generally is most appreciated from a perceptionpoint of view. Considering all requirements, a yellow primary would bethe best choice of an additional primary color in an RGB-display.

FIG. 1 illustrates the sensitivity of the cones in the human eye tolight of various colors. The eye is very sensitive to yellow light (570to 580 nm), which is why adding a yellow primary color to a prior artdisplay with only red, green, and blue primary colors (RGB-display)would substantially improve the overall brightness of a displayed imageand the image quality.

A color other than yellow could nevertheless be a suitable fourthprimary color if images of some special type were to be displayed. Theremay be several applications relating to the field of medical imaging orto the field of printing, wherein the first choice of an additionalprimary color would be a color other than yellow. Although the colorsred, blue, green, cyan, magenta and yellow are mentioned as suitablecolors in preferred embodiments of the invention, this should not beconsidered as a limitation on the invention.

In current color displays (with primary colors red (R), green (G) andblue (B)), the sub-pixels are usually configured according to one of thethree patterns shown in FIGS. 5, 6, and 7.

FIG. 5 is a schematic illustration of the stripe-arrangement ofsub-pixels in prior art color displays. The stripe-arrangement implies asimple array design, simple fabrication procedures and simple drivingcircuitry, but a poor color homogeneity.

FIG. 6 is a schematic illustration of the mosaic-arrangement ofsub-pixels in prior art color displays. The mosaic-arrangement implies asimple array design, and a better color homogeneity, but at the cost ofa more difficult fabrication procedure, and a more complex drivingcircuitry.

FIG. 7 is a schematic illustration of the delta-arrangement ofsub-pixels in prior art color displays. The delta-arrangement impliesthe best color homogeneity and a simple driving circuitry, but at thecost of a more difficult fabrication procedure and a more complex arraydesign.

The stripe arrangement of FIG. 5 is the most popular, followed by themosaic arrangement of FIG. 6 and the delta arrangement of FIG. 7. Theembodiments of the invention will mainly be described with reference tothe stripe arrangement, which should not be considered to be alimitation on the invention, since the invention can be embodied in avariety of display arrangements and display types.

In human perception, homogeneity has an impact on the overall perceivedquality of a display. The problems relating to deficiencies in theperceived homogeneity of a display which the invention seeks to remedyare most readily illustrated in the case of a display with six colors,namely red, green, blue, yellow, magenta and cyan (an RGBYMC-display).FIG. 2 illustrates exemplary chromaticity coordinates of R, G, B, Y, Mand C in a RGBYMC display. FIG. 8 is a schematic illustration of anRGBYMC-stripe-arrangement of sub-pixels in a six-color display. Thedisplay screen comprises a matrix of pixels, which in turn are built upof a repeated arrangement of red (R), green (G), blue (B), yellow (Y),magenta (M) and cyan (C) sub-pixels.

The display is furthermore comprised of several components such as rowand column conductors (not shown), connected to electronics (not shown),such as row and column drivers, all in a manner known by the skilled manand therefore not described here in order not to obscure the inventionin unnecessary detail. If a six-color display is designed in a prior artstripe-arrangement according to FIG. 8, the color homogeneity (colormix) will be very poor.

A much better color homogeneity can be obtained with an alternatingstripe arrangement of RGB- and YMC-stripes according to FIG. 9, whereinRGB-stripes and YMC-stripes alternate in subsequent rows.

An even better embodiment is to alternate RGB-stripes and YMC-stripesbetween subsequent rows and columns according to FIG. 10, whichnevertheless results in approximately the same homogeneity in color asthe previous arrangement.

The distance between two colors, the color distance, denotes thedistance between two colors in the CIE chromaticity diagram, i.e. thelength of a straight line, drawn through the points (chromaticitycoordinates) which represent said two colors. In FIG. 2, the colordistance between green and magenta is illustrated with a broken straightline going from green (G) to magenta (M).

If a first color has the chromaticity coordinates x₁, y₁ in the CIEchromaticity diagram and a second color has the chromaticity coordinatesx₂, y₂ in the CIE chromaticity diagram, one way of calculating the colordistance D_(C) is according to equation (3).D _(C)=√{square root over ((x ₁ −x ₂)²+(y ₁ −y ₂)²)}{square root over((x ₁ −x ₂)²+(y ₁ −y ₂)²)}  (3)

The color distance may nevertheless also be calculated in another way,wherein the human perception of the colors is taken into account, andwherein the color distance is the perceived color distance, either byindividuals or groups of individuals, or in some other way.

The color distance could for instance be calculated in the CIE 1976Yu′v′ color space, wherein u′ and v′ are calculated from X, Y and Zaccording to the equations (4) and (5).

$\begin{matrix}{u^{\prime} = \frac{4\; X}{\left( {X + {15\; Y} + {3\; Z}} \right)}} & (4) \\{v^{\prime} = \frac{9\; Y}{\left( {X + {15\; Y} + {3\; Z}} \right)}} & (5)\end{matrix}$

The color distance between two colors may then be determined accordingto equation (6).D _(C)=√{square root over ((u ₁ ′−u ₂′)²+(v ₁ ′−v ₂′)²)}{square rootover ((u ₁ ′−u ₂′)²+(v ₁ ′−v ₂′)²)}  (6)

According to the invention, the best homogeneity can be obtained byarranging those primary colors that have the maximum color distance(i.e. are most distant from each other in the color space) at a minimalspatial distance from each other (i.e. close to one another), preferablynext to each other, and furthermore by arranging the display elements sothat the sub-pixels of the same colors are equally far apart spatiallyboth in the horizontal direction and the vertical direction. Such asolution would link red to cyan, green to magenta and blue to yellow.

In FIG. 2, the maximum color distance between green (G) and any other ofthe five primary colors, namely the distance between green (G) andmagenta (M) is illustrated as the broken line 21. Analogously, themaximum color distance between blue (B) and any other of the primarycolors, namely the distance between blue (B) and yellow (Y), isillustrated as the broken line 22; and the maximum color distancebetween red (R) and any other of the primary colors, namely the distancebetween red (R) and cyan (C), is illustrated by the broken line 23.

According to a first embodiment of the invention, the display elementsare arranged in an alternating stripe configuration, which would yieldRGB- and CMY-stripes according to FIG. 11.

In the magnified section of FIG. 11, the spatial distances between thered (R) and cyan (C) pixel (111), the spatial distance between the greenpixel (G) and the magenta (M) pixel (112), and the spatial distancebetween the blue pixel (B) and the yellow (Y) pixel (113) areillustrated.

In the embodiment of FIG. 11, said spatial distances 111, 112 and 113are ideally minimized by arranging the display elements next to eachother. The elements may be separated by a small physical distancebetween the elements, as illustrated in the magnified section of FIG.11. According to a second embodiment of the invention, an alternativeconfiguration of RGB- and CMY-stripes, illustrated in FIG. 12, is almostas good as the previous one.

According to a third embodiment, an alternative arrangement of RGBCMY-and CMYRGB-stripes is proposed according to FIG. 13. Since thesub-pixels with the same color are not distributed equally in thehorizontal and vertical directions, this embodiment is not a preferredone.

Using sub-pixel algorithms, the perceived resolution of a display can beincreased substantially. This is already known for conventional RGBdisplays, but similar algorithms may be developed for RGBYMC displays.Such algorithms will perform optimally when white can be created at“near sub-pixel level”. Of all possible arrangements, the arrangement ofFIG. 11 again results in the best performance regarding this aspect,since white can be created with three pixels in the horizontal direction(RGB) or two pixels in the vertical direction (RC, GM, BY) starting fromany sub-pixel in the image. This is in fact better than in RGB mosaicdisplays where three pixels in both the horizontal and the verticaldirections are required. Note that the idea is equally true when thesub-pixel configuration for rows and columns is exchanged.

FIG. 14 is a schematic illustration of the smallest possible whitedisplay element in a prior art RGB display. In the display of FIG. 14, 4rows comprising 5 pixels each, making a total of 20 pixels, areillustrated. Every pixel has red, green and blue sub-pixels, resultingin a total of 60 illustrated sub-pixels.

FIG. 15 is a schematic illustration of the smallest possible whitedisplay element in a display with an RGB/CMY sub-pixel arrangementaccording to an aspect of the invention. In the display of FIG. 15, 3rows comprising 3 pixels each, making a total of 9 pixels, areillustrated. Every pixel has red, green, blue, cyan, magenta and yellowsub-pixels, arranged as in the display of FIG. 11, resulting in a totalof 54 illustrated sub-pixels.

In the conventional RGB display of FIG. 14, white light can be producedby activating a pixel comprising red, green and blue sub-pixels,requiring a total of three sub-pixels in the horizontal direction to beactivated in order to produce white light as indicated by the arrow.Using sub-pixel algorithms, white light can be produced by the displayof FIG. 15 by either activating the red sub-pixel and the cyansub-pixel, the green sub-pixel and the magenta sub-pixel, or the bluesub-pixel and the yellow sub-pixel, as indicated by the arrows. Whitelight can also be produced by activating the red, the green and the bluesub-pixels of a pixel or by activating the cyan, the yellow and themagenta sub-pixels of a pixel.

Although there are three times as many pixels in the RGB display of FIG.14 as in the RGB/CMY display of FIG. 15, the RGB/CMY display of FIG. 15can enable more than 1.5 times the resolution of the RGB display forblack and white images (such as text).

With a view to providing an even better image quality, the arrangementof display elements which are to be arranged next to each other couldcomprise arranging the display elements with the highest luminancesignals at the greatest distance from each other. The principle will beelucidated in the following, with reference to an exemplary RGBYdisplay. This particular display type should however not in any way beconsidered as a limitation on the invention.

The exemplary RGBY display is a regular matrix arrangement comprisingrows and columns of identical pixels, each pixel comprising 2×2sub-pixels (i.e. each pixel comprises four different sub-pixels, one ofevery color, arranged in 2 rows with 2 sub-pixels in every row).

In any RGBY matrix display, the pair of green display elements andyellow display elements typically represent a higher luminance than anyother pair of display elements such as red display elements and bluedisplay elements.

By arranging the green display elements and the yellow display elementson the diagonals of the pixels, instead of on the same row (or column),the spatial distance between the particular pair of display elementswhich represent the highest luminance is maximized.

This can for instance be achieved by arranging the red display elementsand the yellow display elements on a first row, and the green displayelements and the blue display elements on a second row, instead ofarranging the red display elements and the blue display elements on afirst row and the green display elements and the yellow display elementson a second row. In this way, the distance between the centres of theyellow and green display elements would be equal to or larger than anyother spatial distance between any other pair of display elements,namely approximately √{square root over (2)}=1.41 times the distancebetween the green display elements and the blue display elements forinstance.

Hence a new and innovative display which presents the best homogeneityin color and luminance and limits the color and luminance errors andmaximizes the resolution for images comprising black and white text hasbeen proposed.

The illustrated arrangements of the pixels in the displays should not beconsidered to constitute a limitation, since pixels and sub-pixels maybe of various regular or irregular shapes and arranged in a variety ofregular or irregular patterns.

The display according to the present invention may, for example, berealized as a separate, stand-alone unit, or may alternatively beincluded in, or combined with, a mobile terminal for atelecommunications network, such as GSM, UMTS, GPS, GPRS or D-AMPS, oranother portable device of an existing type, such as a Personal DigitalAssistant (PDA), palmtop computer, portable computer, electroniccalendar, electronic book, television set or video game control, as wellas various other office automation equipment and audio/video machinery,etc.

The invention has mainly been described with reference to mainembodiments. However, embodiments other than the ones disclosed aboveare equally possible within the scope of the invention, as defined bythe appended patent claims. All terms used in the claims are to beinterpreted according to their ordinary meaning in the technical field,unless explicitly defined otherwise herein. All references to “a/an/the[element, means, component, member, unit, step etc.]” are to beinterpreted openly as referring to at least one instance of saidelement, means, component, member, unit, step etc. The steps of themethods described herein do not have to be performed in the exact orderdisclosed, unless explicitly specified.

1. A display comprising a plurality of first, second, third, fourth, andfifth sub-pixel display elements, which are arranged in rows andcontrollable by row and column drivers to display first, second, third,and fourth, and fifth different colors, respectively, a first pixel of adisplayed image being reproducible by sequential sub-pixel displayelements in a single row of the display and a second pixel of adisplayed image being reproducible by sequential sub-pixel displayelements in a second single row of the display, said first and secondrows being adjacent and co-planar, wherein for each sub-pixel displayelement associated with the first pixel, there is a minimal spatialdistance between the sub- pixel display element and a respective elementassociated with the second pixel that represents a color having amaximum color distance with respect to the color represented by suchsub-pixel display element compared to the color distance between thecolor represented by the sub-pixel display element and other colors. 2.A display according to claim 1, further comprising sixth sub-pixeldisplay elements, which are controllable to display a sixth differentcolor.
 3. A display according to claim 1, wherein the display is amatrix display.
 4. A display according to claim 1, wherein the differentcolors comprise red, blue, green, yellow and magenta.
 5. The display ofclaim 4, further comprising sixth sub-pixel display elements, which arecontrollable to display a sixth different color, wherein said sixthcolor is cyan.
 6. The display of claim 1, wherein a first sub-pixeldisplay element representing a first color and a second sub-pixeldisplay element representing a second color, said colors representingthe greatest luminance, said first and second sub-pixel display elementsbeing associated respectively with said first and second pixels are botharranged with respect to a third sub-pixel display element representinga third color such that the spatial distance of the first and secondsub-pixel display elements is larger than the spatial distance of eithersaid first or second sub-pixel display element to the third element.